Depends a bit.....
If you have mods (pipe, porting, etc) you may want to get a A/F gauge, but for me I just let the DPM do it's job.
I did opt for a temp gauge, as I do have to ride some trails to get from cabin to backcountry, and the REV get's darn hot without scratchers working full bore. I watch the temp and when it gets high, I know when to dip off-trail, or other......it takes the guesswork out of worrying about temp in hardpack conditions.